Start, in the search box type in cmd and press enter. A black box will pop up, that is the command prompt.
In the command prompt type in sfc /scannow and press enter, windows will then be looking for core components that have been damaged.
Make sure that you are connected to your network, it takes a bit of trial and error.
Malware can also corrupt winsock (the thing that manages internet), in the command prompt type in: netsh winsock reset and press enter and restart the computer.
You can also try powercycling the modem/router, unplug the modem and/or router for 30 seconds, and plug back the modem, wait for 30 more seconds and plug the router back in.
Once everything is back to normal, i recommend that you use Avast! 5 home edition for free, it is a great antivirus/antispyware/antirootkit/program behavior analyzer.